The present invention relates to a sintered carbonitride alloy having titanium as main component intended for use as an insert for turning and milling with improved wear resistance without an accompanying decrease in toughness.
Classic cemented carbide, i.e., based upon tungsten carbide (WC) and cobalt (Co) as binder phase, has in the last few years met with increased competition from titanium-based hard materials, usually called cermets. In the beginning these titanium-based alloys were based on TiC+Ni and were used only for high speed finishing because of their extraordinary wear resistance at high cutting temperatures. This property depends essentially upon the good chemical stability of these titanium-based alloys. The toughness behavior and resistance to plastic deformation were not satisfactory, however, and therefore the area of application was relatively limited.
Development proceeded and the range of application for sintered titanium-based hard materials has been considerably enlarged. The toughness behavior and the resistance to plastic deformation have been considerably improved. This has been done, however, by partly sacrificing the wear resistance.
An important development of titanium-based hard alloys is the substitution of carbides by nitrides in the hard constituent phase. This decreases the grain size of the hard constituents in the sintered alloy. Both the decrease in grain size and the use of nitrides lead to the possibility of increasing the toughness at unchanged wear resistance. Characteristic for said alloys is that they are usually considerably more fine-grained than normal cemented carbide, i.e., WC-Co-based hard alloy. Nitrides are also generally more chemically stable than carbides which results in lower tendencies to stick to work piece material or wear by solution of the tool, the so-called diffusion wear.
In the binder phase, the metals of the iron group, i.e., Fe, Ni and/or Co, are used. In the beginning, only Ni was used, but nowadays both Co and Ni are often found in the binder phase of modem alloys. The amount of binder phase is generally 3-25% by weight.
Besides Ti, the other metals of the groups IVa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are normally used as hard constituent formers as carbides, nitrides and/or carbonitrides. There are also other metals used, for example Al, which sometimes are said to harden the binder phase and sometimes improve the wetting between hard constituents and binder phase, i.e., facilitate the sintering.
A very common structure in alloys of this type is hard constituent grains with a core-rim-structure. An early patent in this area is U.S. Pat. No. 3,971,656 which comprises Ti- and N-rich cores and rims rich in Mo, W and C.
It is known through U.S. patent application Ser. No. 07/543,474 (our reference: 024000-757), which is herein incorporated by reference, that at least two different combinations of duplex core-rim-structures in well-balanced proportions give optimal properties regarding wear resistance, toughness behavior and/or plastic deformation.
When using inserts of sintered carbonitride in turning and milling, the inserts are worn. On the rake face (that is, that face against which the chips slide) so-called crater wear is obtained where the chip comes in contact with the insert. In connection herewith, a crater is formed which successively increases in size and gradually leads to insert failure. On the clearance face, that face which slides against the work piece, so-called flank wear is obtained which means that material is worn away and the edge changes its shape. A characteristic property for titanium-based carbonitride alloys compared to conventional cemented carbide is the good resistance against flank wear. Decisive for the tool life is therefore most often crater wear and how this crater moves toward the edge whereby finally crater breakthrough takes place which leads to total failure.